The recent advent of failure mode testing systems to activate failure modes has enabled manufacturers to discover latent defects or flaws which may eventually lead to the failure of a product, component or sub-component. The methodology of these testing systems generally involves the application of one or more types and/or levels of stimuli to the product, component or sub-component until one or more failure modes are activated. Typically, one of the stimuli includes vibration, such as that caused by striking a piston, rod, or other suitable device, against the product, component or sub-component itself, or a surface in contact with the product, component or sub-component. When the failure mode is activated, the failed product, component or sub-component is then either repaired, replaced, or redesigned. This process may then be repeated in order to activate and eliminate other failure modes.
A proprietary testing system has been developed by Entela, Inc. (Grand Rapids, Mich.) and is referred to as a failure mode verification testing system. This system, which is described in commonly-owned U.S. patent application Ser. No. 09/316,574 entitled “Design Maturity Algorithm”, filed May 21, 1999, now U.S. Pat. No. 6,247,366; and U.S. patent application Ser. No. 08/929,839 entitled “Method and Apparatus For Optimizing the Design of Products”, filed Sep. 15, 1997, now U.S. Pat. No. 6,035,715, employs an apparatus which is capable of generating a wide variety of stress patterns, especially six axis uniform random stress patterns, in a product, component, or sub-component in order to activate the failure modes of that particular product, component, or sub-component.
A six axis uniform random stress is generally defined as the stress history at a point having uniform random distribution with the stress being comprised of tension and compression stress in three orthogonal axes and torsion stress about the same three orthogonal axes. Six axis uniform random stress patterns are generally defined as six axis uniform random stress at all points on a product such that the stress history of the six axis uniform random stress at each point forms a time history of non-repeating stress patterns.
The apparatus uses six axis uniform random actuation at one or more mounting locations of a product to produce six axis uniform random stress patterns in the product. These six axis uniform random stress patterns identify failure modes previously uncovered with other testing methodologies. Furthermore, the simultaneous introduction of other stimuli (at varying levels), such as temperature, vibration, pressure, ultraviolet radiation, chemical exposure, humidity, mechanical cycling, and mechanical loading, identify other failure modes associated with the product.
In order to create the six axis uniform random stress patterns in the product, the apparatus employs a plurality (preferably six) of actuators, also referred to as force imparting members, that can be operated either pneumatically, hydraulically, by a combination of both pneumatic and hydraulic power, or any other force imparting mechanism. A portion of the actuators, such as the cylinders, are typically mounted (including slidingly), either directly or indirectly, onto one or more support members.
If six actuators are being used, they are preferably arranged in pairs, each pair being set about 120 degrees apart from the other pair. Each actuator is simply comprised of a cylinder acting in cooperation with a piston in order to produce force and torque upon a point of rotation. The pressure to each actuator is preferably cycled between maximum extend pressure and maximum retract pressure in a linear “saw-tooth” manner. The frequency for each actuator is slightly different. This difference in frequency causes an interference pattern of the cycling as the actuators come in and out of phase with each other. It is this difference in the frequencies of the actuators which creates a six axis uniform random stress in the product. By way of a non-limiting example, the six pneumatic actuators can be operated at frequencies of 1.8 Hz, 1.9 Hz, 2.0 Hz, 2.1 Hz, 2.2 Hz, 2.3 Hz, and 2.4 Hz, respectively. Therefore, as the actuators come in and out of phase with one another, the frequency content in the center will go from about 2 Hz to infinity. It should be noted that other frequencies may be used for the individual actuators in order to produce an even lower frequency.
A portion of the actuators, such as the pistons, are typically connected, either directly or indirectly, to a platform, such as a hub, upon which the product is mounted. As the actuators are actuated, they produce a force which generates a torque about a point of rotation on the platform. It should be noted that whether a torque is generated about the point of rotation will depend upon which actuators are being actuated and in what sequence with respect to one another. The force and torque are eventually transferred from the platform to the product itself, thus creating the six axis uniform random stress patterns in the product.
During routine operation of the apparatus, it is not uncommon for the actuators to be cycled back and forth very rapidly. Therefore, it is impractical to manually attempt to continuously adjust the various operational parameters that affect actuator operation, such as the pressure and frequency of the cylinders. The pressure parameter concerns the amount of pressure in the air line (e.g., in a pneumatic system) in communication with the cylinder of the actuator, which is typically expressed in pounds per square inch (psi). The frequency parameter concerns the frequency that each cylinder is set to, which is typically expressed in Hertz (Hz).
By way of a non-limiting example, the system response of the apparatus can be measured in terms of energy E (e.g., grms or peak G) and slope m of the fast Fourier transform (FFT) of the system response. A FFT is typically performed on a time history or a response. By way of a non-limiting example, an acceleration signal from an accelerometer would provide a varying signal in time. The FFT of the acceleration signal would give the acceleration level vs. frequency. From the FFT of the acceleration signal, the slope of the FFT plot (i.e., response level vs. frequency) can be determined.
Preferably, a desired energy level E having a desired slope m (e.g., flat) is produced by the application of appropriate levels of pressure and frequency. For example, if the energy level were plotted on the Y-axis of a graph and the frequency level were plotted on the X-axis of that same graph, the majority of data points could be bisected by a line having a slope substantially equal to zero. Thus, the energy level would be substantially constant over the entire frequency range.
Due to the large number of calculations that would have to be performed on a split second basis for each of the six actuators, it is impractical to manually perform the calculations, let alone make the requisite adjustments to the operational parameters of the actuators, without adversely affecting the efficient performance of the testing system. Nonetheless, it is important to the optimal operation of the testing system that the desired performance parameters are achieved and maintained during the course of the testing procedure.
Additionally, with respect to pressure, it has been observed that by keeping each cylinder at a constant pressure, the actuators have a tendency, due to frictional forces and historesis, to gravitate towards a set point and get stuck, thus causing the actuators to improperly function. For example, if the pressure is slightly too high, the apparatus will tend to drift up and then get stuck. Conversely, if the pressure is slightly too low, the apparatus will tend to drift down and then get stuck.
Furthermore, with respect to frequency, it has been observed that by keeping each cylinder at a specific individual frequency, over time one of the actuators will receive less energy than the other actuators. For example, if one cylinder of an actuator pair is operated at 5 Hz at a given pressure, and the other cylinder of the actuator pair is operated at 5.5 at that same given pressure, then over time a little more energy is being given to one cylinder than the other. The fact that one cylinder receives more energy can be confirmed by calculating the integration of the given pressure at the slightly higher frequency. As a result, the apparatus will tend to drift toward the actuator having the lowest energy level, thus causing performance problems.
Therefore, there exists a need for a control system for determining if the desired system response of a failure mode testing system is or is not present. The control system should be capable of quickly, accurately, and if needed, constantly adjusting the operational parameters (e.g., pressure and frequency) until the desired system response is achieved and subsequently maintained.